The invention relates generally to goniophotometers and integrating spheres for photometry, radiometry, spectroradiometry and/or colorimetry to measure the luminous intensity and luminous flux from various luminaires. More particularly, it relates to a method and apparatus for measuring the luminous intensity from a luminaire as a function of the viewing angle.
In the field of lighting system design it is necessary to know the luminous intensity emitted by a luminaire at all viewing angles from the luminaire. In particular, luminaire's using light emitting diodes (LEDs) have custom-made lenses and reflectors designed to create a specific, controlled lighting distribution that is not homogeneous. Whenever a large area is to be illuminated and more than a single luminaire is required, the luminous intensity of the light emitted by each luminaire at various angles must be determined to assure the entire area is sufficiently illuminated. Thus, the system design requires a measurement device and method to determine the luminous intensity of the light emitted at different angles from a luminaire, and the total luminous flux emitted from the luminaire. Furthermore, luminaire vendors must specify these values for each of their products. Luminaire customers and regulatory agencies use the published values to compare different products, and ensure that the products meet required specifications. There is therefore a requirement for an apparatus and method to accurately measure the luminous intensity distribution and luminous flux of a luminaire.
The currently accepted device for determining the lumninous intensity as a function of angle from the luminaire is a goniophotometer. This apparatus uses a single photodetector (for total light) or spectrophotometer (for color information). FIG. 1 shows one embodiment of the prior art goniometer wherein a luminaire 101 is mounted on a rotating mount 102 that is rotated around the horizontal axis 103 relative to the luminaire. The light from the luminaire is reflected off a rotating mirror 113 that is rotated around the vertical axis 104 relative to the luminaire. A spectrophotometer 106 behind an aperture 105 measures the light at the set horizontal and vertical angles. The goniophotometer is located inside a dark room 100 to prevent stray or reflected light from distorting the measurements. The goniophotometer measures the luminous intensity received on the detector at a given distance from the luminaire as the angle of viewing is varied by rotating the luminaire mount or mirror. In some implementations, one or two rotating mirrors between the luminaire and detector are used to vary the angle. The luminous flux can be obtained by integrating the goniophotometer results over a reference sphere surrounding the luminaire. A common aspect of the prior art is that a single detector is used to measure the luminous intensity at separate angles while the angle of the detector relative to the luminaire is varied by mechanical means. The use of a goniophotometer required for official characterization of a luminaire can be traced to National Institute of Standards and Technology (NIST) standards with measurements done at Nationally Recognized Test Laboratories (NRTLs).
A goniophotometer has two significant limitations for commercial applications. A first significant limitation is the time required for measurement. The precise mechanical rotation together with the time required for a measurement with a single detector means that several hours of measurements are required to characterize one luminaire. A second disadvantage is cost. A large-scale goniophotometer requires large mechanical mounts with precision controls, resulting in a complete system costing on the order of $100,000.
These limitations make it difficult to quickly characterize luminaires at a reasonable cost during a lighting system design cycle. For example, if it is desired to compare several different LED lenses on a luminaire, each lens must be installed on the LEDs in the luminaire, the luminaire installed in goniophotometer, and then several hours of measurements taken for each lens.
There is therefore a need for a system and method to quickly characterize and compare luminaire designs during the design phase, without necessarily requiring NIST accuracy. The system presented here provides a low-cost, real-time measurement system and method to characterize luminaires.
Of interest in the prior art are patents on goniphotometers (U.S. Pat. No. 7,800,745 and U.S. Pat. No. 5,949,534 for example), integrating spheres (U.S. Pat. No. 6,721,048 for example), commercially available digital lux sensors from Intersil (part number ISL29023) and digital color sensors from AMS (part number TCS3472), and systems of networked sensors (U.S. Pat. No. 8,587,414 for example).